1. Field of the Invention
Embodiments of the present invention generally relate to seismic surveying and, more particularly, to a method for attenuating multiples in seismic data.
2. Description of the Related Art
The following descriptions and examples do not constitute an admission as prior art by virtue of their inclusion within this section.
Seismic surveying is a method for determining the structure of subterranean formations in the earth. Seismic surveying may typically utilize seismic energy sources which generate seismic waves and seismic receivers which detect seismic waves. The seismic waves may propagate into the formations in the earth, where a portion of the waves may reflect from interfaces between subterranean formations. The seismic receivers may detect the reflected seismic waves and convert the reflected waves into representative electrical data. The seismic data may be transmitted by electrical, optical, radio or other means to devices which record the data. Through analysis of the recorded seismic data (or seismograms), the shape, position and composition of the subterranean formations may be determined. Such analysis may indicate the presence or absence of probable locations of hydrocarbon deposits or other valuable substances.
Depending on the location where a survey takes place, there are surveys in sea, on land or in transition zones. Marine seismic surveying is a method for determining the structure of subterranean formations underlying bodies of water. Marine seismic surveying may typically utilize seismic energy sources and seismic receivers located in the water which may be either towed behind a vessel or positioned on the water bottom from a vessel. The energy source may typically be an explosive device or compressed air system which generates seismic energy, which then propagates as seismic waves through the body of water and into the earth formations below the bottom of the water. As the seismic waves strike interfaces between subterranean formations, a portion of the seismic waves may reflect back through the earth and water to the seismic receivers, to be detected, transmitted, and recorded. The seismic receivers typically used in marine seismic surveying may be pressure sensors, such as hydrophones. Additionally, motion sensors, such as accelerometers, may be used. Both the sources and receivers may be strategically repositioned to cover the survey area.
Land seismic surveying is done on land. The energy sources are typically vibratory sources (vibrators). The vibrators produce a pressure signal that propagates through the earth into the various subsurface layers. Here elastic waves are formed through interaction with the geologic structure in the subsurface layers. Elastic waves are characterized by a change in local stress in the subsurface layers and a particle displacement, which is essentially in the same plane as the wavefront. Acoustic and elastic waves are also known as pressure and shear waves. Acoustic and elastic waves are collectively referred to as the seismic wavefield.
A reflected wavefield may consist of both primary reflections and multiple reflections. Primary reflections may be defined as seismic waves which have reflected only once, from an interface between subterranean formations, before being detected by a seismic receiver. Primary reflections contain the desired information about the subterranean formations which is the goal of marine seismic surveying. Multiple reflections, or multiples, may be defined as seismic waves which have reflected more than once before being detected by a seismic receiver.
FIG. 1 illustrates two types of multiples: internal multiple 110 and a surface multiple 120. A “surface multiple” is herein defined as any seismic event that is generated by at least two upward reflections and at least one downward reflection from the free surface boundary. The free surface boundary is typically the sea surface in marine environment, or earth surface in land surveys.
Another type of multiples is an “internal multiple”, which is herein defined as any seismic event that is generated by at least two upward reflections and at least one downward reflection from a boundary below the free surface with no downward reflection from the free surface. The internal multiples comprise multiple reflections between reflectors and media within the earth subsurface, whose physical properties are unknown and usually need to be determined by the survey.
In the context of seismic surveying, multiple attenuation is a pre-stack inversion of a recorded wavefield, which aims at removing the energy associated with multiple reflections. Theoretically, multiple attenuation can be pursued in a totally data-driven manner by evaluating equations which involve continuous summations over the directions of space.
Surface Multiples
There are several methods available to attenuate surface related multiples. Wavefield extrapolation techniques that estimate surface multiples have been described in many publications. Wiggins (1999) summarized the 2D technique that subtracts forward extrapolated shot gathers from backward extrapolated shot gathers. In this case, the primaries of the forward extrapolated shots matched the first order surface multiples of the backward extrapolated shots. Subtracting these data sets and forward extrapolating the results removed the first order surface multiples from the input shot gathers.
Kabir et. al. (2004) extended Wiggins' method to 3D and used it to attenuate water-bottom multiples and peg-legs. But this method required receiver line interpolation to ensure adequate crossline data coverage.
Pica et. al. (2005) extended the 3D wavefield extrapolation technique to essentially de-migrate a depth image using a given velocity field to compute primaries and surface multiples. The first de-migration process resulted in an estimate of the primaries, and subsequent de-migrations computed successively higher orders of surface multiples. They indicated that a recorded shot can be used in place of the computed primary wavefield.
Stork et. al. (2006) used a similar de-migration scheme to compute primaries and surface multiples. This method is called Wavefield Extrapolation Multiple Modeling (WEMM), and performs the following four steps. FIGS. 6-9 illustrate the steps and a four-layer model 600, which has three interfaces 611, 612 and 613, and a bottom 614.
Step 1: Using wavefield extrapolation, forward propagate a point source 620 at the source location (S, considered the surface) downward through the given reflectivity and velocity models 600, and compute and store upward reflection information (a 628, b 627 and c 626 in FIG. 6). Stop the propagation at a specified depth (bottom 614).
Step 2: Forward propagate a 2D zero energy wavefield from the bottom 614 of the given reflectivity and velocity models 600 to the surface 610, and accumulate all upward reflection information previously computed (c′ 636, b′ 637 and a′ 638 in FIG. 7). Save the 2D wavefield at the surface 610. This wavefield consists of primary reflections generated from one upward bounce off of the modeled reflectors 611, 612 and 613.
Step 3: Forward propagate the recorded 2D wavefield at the surface downward through the given reflectivity and velocity models 600, and again compute and store upward reflection information (d 648, e 647 and f 646 in FIG. 8). Stop the propagation at the bottom 614.
Step 4: Forward propagate a 2D zero energy wavefield from the bottom 614 of the given reflectivity and velocity models 600 to the surface 610, and accumulate all upward reflection information previously computed (d′ 658, e′ 657 and f′ 656 in FIG. 9). Save the wavefield at the surface 610. This wavefield consists of first order surface multiples generated from two upward reflections off of the modeled reflectors and one downward bounce off of the free surface 610.
Higher orders of surface multiples can be computed by increasing the number of propagations performed. Surface multiples of order 1 to N−1 are computed from N propagations.
Internal Multiple Prediction
Pica and Delmas (2008) used a wavefield extrapolation method similar to the surface multiple prediction method described by Pica et. al. (2005) to compute 3D internal multiples from marine data. Their method consists of four separate extrapolation steps.
Step 1: Back propagate recorded shots to an arbitrary depth.
Step 2: Upward propagate the back propagated shot through the migrated image to generate a first set of secondary sources at reflectors.
Step 3: Propagate the wavefield generated from first set of secondary sources downward through migrated image, building a second set of secondary sources at the reflectors.
Step 4: Propagate the wavefield generated from the second set of secondary sources upward through migrated image, resulting in an internal multiple model.
This method of internal multiple modeling inputs recorded shots as the initial wavefield, and adds to all events in the shot of first order internal multiple whose generating horizon lies above the arbitrary depth established in Step 1.
As described above, the prior art methods have some success in predicting and attenuating multiples. But it is still not satisfactory for many situations. Internal multiples are still difficult and costly to remove from seismic data, especially as the dimension for the problem increases. It is desirable to have methods to effectively attenuate various multiples, especially internal multiples.